Mi-8 & Mi-24 main rotor Swash Plate wrong motion

[GumidekCZ]

Simply, the swash plate is controlled so its tilted 90° precceding the tilt of rotating blade disc due to compensation of Gyroscopic precession effect.

!!! EDIT: NOT EXACTLY TRUE - SOLUTION IS DESCRIBED BY ME IN THE TOPIC DOWN BELLOW !!!

In DCS the movement of swash plate and rotor blade disc is in same plane - WRONG.
SIKORSKY_HELICOPTER_FLIGHT_THEORY FOR PILOTS AND MECHANICS - ADA119096 - starting at page 27

Mi8_swash_plate_BUG.trk

Mi-24_swash_plate_BUG.trk

In the video bellow, when the swash plate tilted to the side, the blade disc in flight will be tilted forward or aft - controlling the PITCH.

Edited December 4, 2023 by GumidekCZ

[AeriaGloria]

1 hour ago, GumidekCZ said:

Swash plate never have 90 degree tilt offset because part of 90 degree angle will account to pitch horn angle offset. At Apache and Blackhawk it's exactly 45 degree swash plate +45 degree pitch horn.

Gyroscopic momentum Always act in axis perpendicular to rotation and force applied.

The swash plate have most common tilt offset of 180 divided by # of blades. Look at 3, 4 or 5 blades rotors and you will see that it's matches.

Bug is preliminary confirmed by Mi-8(Mi-171š) pilot.

 

Since you asked to have the other thread moved to Mi-8 section I’ll reply here. 
 

You said swashplate should have tilt by number of blades, 180 divided by 5 is 36 degrees. 
 

And plus the pitch horn offset, equals the angle from which the blade pitch is angled from say, direct forward for a backwards pitch. 

Now if I’m translating the technical document on the swashplate correctly, it says that the angle of the pitch lever horn to the horizontal flapping hinge is 72 degrees.
 

However, it also mentions that the tangent of this angle is the coefficient of the swing compensator. Is the swing compensator the angle we are looking for? In this case, the tangent would be about 172.3 degrees. 
 

That is unless it means the opposite angle, 28 degrees, which would be 
a tangent coefficient of about 30.4 degrees. 
 

Not sure if any of these numbers help, or if your Mi-8/171 pilot gave you specific numbers 

 

[DeMonteur]

On 12/2/2023 at 3:13 PM, AeriaGloria said:

Since you asked to have the other thread moved to Mi-8 section I’ll reply here. 
 

You said swashplate should have tilt by number of blades, 180 divided by 5 is 36 degrees. 
 

And plus the pitch horn offset, equals the angle from which the blade pitch is angled from say, direct forward for a backwards pitch. 

Now if I’m translating the technical document on the swashplate correctly, it says that the angle of the pitch lever horn to the horizontal flapping hinge is 72 degrees.
 

However, it also mentions that the tangent of this angle is the coefficient of the swing compensator. Is the swing compensator the angle we are looking for? In this case, the tangent would be about 172.3 degrees. 
 

That is unless it means the opposite angle, 28 degrees, which would be 
a tangent coefficient of about 30.4 degrees. 
 

Not sure if any of these numbers help, or if your Mi-8/171 pilot gave you specific numbers 

 

I cannot give @GumidekCZ specific numbers since I don't remember them because it is not important for me. All I know is gyroscopic precession is not compensated by swashplate because it is not needed. I need to compensate for gyroscopic precession only when I am entering too quickly to steep turn (45 degree turn). If I increase bank about 10deg/s, gyroscopic precession is too small to have any noticeable effect. 

Please try to find the exact numbers If you have them somewhere

[AeriaGloria]

16 hours ago, DeMonteur said:

I cannot give @GumidekCZ specific numbers since I don't remember them because it is not important for me. All I know is gyroscopic precession is not compensated by swashplate because it is not needed. I need to compensate for gyroscopic precession only when I am entering too quickly to steep turn (45 degree turn). If I increase bank about 10deg/s, gyroscopic precession is too small to have any noticeable effect. 

Please try to find the exact numbers If you have them somewhere

True, the manual does mention 

If you pitch up, aircraft will want to roll left

 If you pitch down, aircraft will want to roll right

 If you roll right, aircraft will want to pitch up

 If you roll left, aircraft will want to pitch down

While these are much smaller then the intended movement, it does seem that it doesn’t completely compensate, but only mostly.

Edited December 4, 2023 by AeriaGloria

[GumidekCZ]

I´ve got it!

I have found explanation in Czech Technical University Bachelor thesis (Google translator or any other comes handy):
https://core.ac.uk/download/pdf/30291859.pdf
There is an angle between axis of (flapping) horizontal hindge and pitch horn (point A) - in picture angle designated as Delta
According to Mi-8 aerodynamic manual 0,5 = tan(Delta) , so the Delta = 26,6°
This angle decrease desired 90° angle. As measured by myself from drawing, it equals roughly to 36°
So well known Mi-8 swash plate offset angle 21°(Delta) + Angle of pitch horns 36° (Tau) + above described lag angle 26,6° (Sigma) = 83° almost our desired 90°
Note: my angular measure were dony only on my laptop screen with help of some tools and probably have some errors.

SWASH PLATE BUG - fix axis of movement for lateral and longitudal tilt by 21° CCW. Keep rotor disc animation as it is now.
Mi-24 Hind have identical main rotor hub and swash plate machinsm with same angles as described above.

 

Edited December 5, 2023 by GumidekCZ

[AeriaGloria]

1 hour ago, GumidekCZ said:

I´ve got it!

I have found explanation in Czech Technical University Bachelor thesis (Google translator or any other comes handy):
https://core.ac.uk/download/pdf/30291859.pdf
There is an angle between flapping pin and pitch horn (point A) - in picture angle designated as Delta
This angle decrease desired 90° angle. As measured by myself from drawing, it equals roughly to 36°
So well known Mi-8 swash plate offset angle 21°(Delta) + Angle of pitch horns 36° (Tau) + above described lag angle 26° (Sigma) = 83° almost our desired 90°
Note: my angular measure were dony only on my laptop screen with help of some tools and probably have some errors.

SWASH PLATE BUG - fix axis of movement for lateral and longitudal tilt by 21° CCW. Keep rotor disc animation as it is now.
I think that Mi-24 Hind have same 21° - NOT confirmed yet - searching for evidence. Than the fix would be the same as for Mi-8.

 

 

Where did the 21 degree come from? 

[GumidekCZ]

From the swash plate actuators position.

Red - pitch control
Blue - transversal (roll) control

Both channels works independently on each other. Not like in DCS now, where if you push cyclic forward - both with actuate = wrong.

Edited December 4, 2023 by GumidekCZ

[GumidekCZ]

   in attached doc on page 28 (this graphic not representing Mi-8 control)
Рис. 8.18. Угол опережения автомата перекоса:
Величина угла опережения автомата перекоса у разных вертолетов различна и находится в пределах 20-26° (у вертолета Ми-8 dАП =21°).
При отклонении ручки продольно-поперечного управления, например, вперед наружное кольцо с тарелкой ввиду наличия угла опережения отклонится вперед и влево, а конус вращения несущего винта в результате циклического изменения установочного угла лопастей отклонится строго вперед.
Translation:
Fig. 8.18. The advance angle of the automatic skew control unit:
The value of the advance angle of the automatic tilt control unit varies from helicopter to helicopter and is in the range of 20-26° (Mi-8 helicopter dAP =21°).
When the longitudinal-transverse control handle is deflected, for example, forward, the outer ring with the plate, due to the presence of an advance angle, will deflect forward and to the left, and the cone of rotation of the main rotor will deflect strictly forward as a result of a cyclic change in the installation angle of the blades.
Source:
8. Управление вертолетом Ми-8.doc

Edited December 4, 2023 by GumidekCZ

[GumidekCZ]

Better drawing, with Mi-8/24 offset angle of wash plate precceding control ΔΨупр = 21°.

Edited December 5, 2023 by GumidekCZ

[GumidekCZ]

Drawing from which I measured the angle of pitch horns 36° (Tau) and falp stiffening angle 26° (Sigma)

HOW THE ANGLE σ1 BETWEEN PITCH HORN AND HORIZONTAL (FLAPPING) HINDGE IS AFFECTING THE BLADE ANGLE ф :
(ΔΨупр - is the offset angle of swash plate control = for Mi-8/24 it is the 21° )
Угол  опережения  изменения  циклического  шага  лопасти  определяется коэффициентом  компенсатора взмаха лопасти (к),  углом  поводка  лопасти  σ11 и  средним  углом  поворота  (отставания)  С0  лопасти  относительно  вертикального шарнира [6]:    ΔΨупр =< σ11 - С0 -  arctg(k)  (1)
Компенсатор  взмаха  в  системе  управления  лопастями  несущего  винта (рис.29) -  это особая кинематическая связь угла установки лопасти  ф  от угла взмаха лопасти  β НВ при ее вращении в своей плоскости, обеспечивающая автоматическое уменьшение угла установки лопасти при взмахе вверх и увеличение его при опускании лопасти.  Это нужно для регулирования и ограни чения взмаха лопасти в плоскости ее вращения.

 

DeepL translation:
The advance angle of the blade cyclic pitch change is determined by the blade sweep compensator coefficient (k), the blade leash angle  σ11 and the average angle of rotation (lag) С0  of the blade relative to the vertical joint [6]:     ΔΨупр =< σ11 - С0 -  arctg(k)  (1)
The blade sweep compensator in the main rotor blade control system (Fig.29) is a special kinematic link between the blade setting angle ф and the blade sweep angle β HB during its rotation in its plane, which provides automatic reduction of the blade setting angle during upward sweep and its increase during blade descent.  This is necessary to regulate and limit the blade sweep in the plane of rotation.

Edited December 5, 2023 by GumidekCZ

[GumidekCZ]

Well, this document for Mi-8 describes in more detail everything I tried to explain above.

Aerodynamics_Mi8.docx

Особенностью работы НВ является инерционность его лопастей. Поэтому начало взмаха лопасти отстаёт от начала изменения угла установки на угол азимута приблизительно 90° −σк, где σк − характеристика регулятора взмаха. Это создаёт неудобство управления, так как не обеспечивается независимость каналов управления  вертолетом.
     Конструкция системы  продольно- поперечного управления вертолетом включает в себя так называемое опережение управления, назначение которого:
     -обеспечить полное соответствие наклона равнодействующей НВ Rн отклонению ручки циклического шага (РЦШ);
     -исключить взаимовлияние продольного и поперечного каналов управления, вызванное инерционностью лопастей.
     Опережение обеспечивается: смещением пальцев крепления тяг продольно-поперечного управления на 21° против вращения НВ (рис. 32), а также подбором характеристики регулятора взмаха (угла σ).

TRANSLATED:
A special feature of the main rotor operation is the inertia of its blades. Therefore, the beginning of blade sweep lags behind the beginning of attitude angle change by an azimuth angle of approximately 90° -σk, where σk is the characteristic of the sweep regulator. This creates inconvenience control, as it does not provide independence of helicopter control channels.

The design of the system of longitudinal and transverse control of the helicopter includes the so-called control advance, the purpose of which:
     - provide a full correspondence of the slope of the equinoctial force of main rotor disc Rn to the deviation of the cyclic pitch stick;
     - exclude mutual influence of longitudinal and transverse control channels caused by blade inertia.

The advance is provided by: shifting the longitudinal-transverse control linkage pins by 21° against main rotor rotation (Fig. 32), as well as by selecting the sweep regulator characteristic (angle σ).

Fig. 32 Control advance mechanism